Packaging DNA during formation of spermatozoa. During transformation of spermatids into spermatozoa at terminal stages of spermtogenesis, chromosomal DNA becomes tightly packaged as a result of the replacement of somatic-type histones by sperm histone. A model experiment using calf thymus chromatin and salmon protamine has shown that the chromosomally-associated protease could fulfill the task of displacing slightly lysine-rich and arginine rich histones in combinations with acetylation of these histones, while displacement of lysine-rich histone can be explained on the basis of its decreased affinity to DNA as a result of saturated binding of sperm histone to spermatid chromatin. Unpacking of sperm chromatin during fertilization. During fertilization tightly packaged sperm chromatin is dispersed after incorporation of the sperm head into the egg cytoplasm. When isolated bull sperm chromatin is incubated under appropriate conditions, an extensive proteolytic degradation of sperm histone occurs, being accompanied by a marked swelling of the chromatin masses. The protease found in isolated bull sperm chromatin possesses properties indistinguishable from those of acrosin. Evidence suggests that the protease travels along chromatin strands and hydrolyzes essentially all the sperm histone moelcules within the chromatin masses. Proteolysis of sperm histone may be key events during transformation of the sperm nucleus into the male pronucleus in the egg cytoplasm. BIBLIOGRAPHIC REFERENCES: Wong, T.K. and Marushige, K. Modification of histone binding in calf thymus chromatin and in the chromatin-protamine complex by acetic anhydride. Biochemistry 15(1976) in press. Marushige, K., Marushige, Y. and Wong, T.K. Complete displacement of somatic histones during transformation of spermatid chromatin: A model experiment. Biochemistry 15(1976) in press.